Paper mills have for many years made extensive use, for the cleaning of paper making stock, of pressure screening apparatus embodying a cylindrical perforated screening member which defines screening and accepts chambers on the opposite sides thereof in a closed housing, and wherein a rotor member operates in one of the chambers to keep the screening perforations open and free from solid material having a tendency to cling to the surface of the screening cylinder.
The assignee of this invention has manufactured and sold many such screens in accordance with a series of U.S. patents, commencing with Staege U.S. Pat. No. 2,347,716, and followed by Martindale U.S. Pat. No. 2,835,173 and numerous other patents including Seifert U.S. Pat. Nos. 3,849,302 and 4,105,543. In operation, the stock or furnish is delivered to the screening chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from the opposite end of the screening chamber. In some cases, heavy reject material is prevented from entering the screening chamber so that only light reject material is removed after passing through the screening chamber, as shown in Martin-Sauzedde U.S. Pat. No. 4,851,111.
Starting with the construction shown in the above Martindale patent, all such screens manufactured and sold by applicants' assignee have been characterized by a rotor which included bars or vanes of airfoil section moving in closely spaced but non-contacting relation with the surface of the screening cylinder for the purpose of creating alternating positive and negative pressure waves or pulses effective on the perforations in the screening cylinder to prevent plugging thereof. Applicants' assignee has experimented to a considerable extent with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, some such variations being shown in Seifert-Chupka U.S. Pat. No. 3,970,548, Chupka-Seifert U.S. Pat. No. 4,328,096 and Chupka et al U.S. Pat. No. 4,663,030.